This work focuses on the molecular mechanisms of fibrillation. It is motivated by the recent discovery of two different channelopathies associated with gain-of-function mutations in the KCNJ2 gene that codes for the inward rectifier channel (Kir2.1) protein responsible for IK1: i) a variant of short QT syndrome, SQTS3, resulting from a mutation (D172) that increases risk of sudden cardiac death;and ii) a new type of familial atrial fibrillation (AF) that results from a different mutation (V93I) in KCNJ2. Our Specific Aims are: 1. To test the hypothesis that the outward component of IK1 is critical in controlling frequency and stability of rotors responsible for fibrillation. We will use computer simulations of 2D propagation and experiments in adult hearts and neonatal myocyte monolayers from D172N and V93I mutant mice, mice overexpressing wild-type Kir2.1 channels, and Kir2.1-AAA mice. We will also determine the effects of changing [K+]o on frequency and stability of reentry and fibrillation. Simulations of human cardiac excitation will help us check the clinical relevance of our results. 2. To compare the effects of increasing versus decreasing IK1 in the mechanism of sink-to-source mismatch leading to wavebreak and reentry. Simulations and experiments in monolayers will compare the effects of D172N and V93I mutations with those of increasing or decreasing the expression of Kir2.1 channels on excitability, curvature-velocity relationships and vortex-shedding. We will test the hypothesis that gain-of-function of Kir2.1 increases the critical radius of curvature for successful propagation and thus the incidence of wavebreaks, whereas loss-of function has the opposite effects. 3. To determine the role of spatial gradients in the expression of wildtype and mutant Kir2.1 proteins on wavebreak formation and reentry. We hypothesize that gradients in IK1 density contribute to dispersion of refractoriness, and Kir2.1 overexpression amplifies the arrhythmogenic effect of dispersion. Numerical and biological experiments in wildtype and genetically altered myocyte monolayers having specific patterns of IK1 density dispersion will ascertain the role of Kir2.1 channel gradients in reentry. Also, immunohistochemistry, patch clamping and optical mapping in adult hearts will determine whether Kir2.1 gradients contribute to wavebreak and reentry in the intact mouse heart. Altogether, these studies should provide insights into mechanisms of arrhythmias in SQTS3 and AF patients, and possibly also in many patients affected by idiopathic VF.